<div id="map"></div>

<script src="//d3js.org/d3.v3.min.js" charset="utf-8"></script>
<script>
    var map = new mapboxgl.Map({
        container: 'map',
        style: 'mapbox://styles/mapbox/satellite-v9',
        zoom: 0
    });

    map.on('load', function() {
        // We use D3 to fetch the JSON here so that we can parse and use it separately
        // from GL JS's use in the added source. You can use any request method (library
        // or otherwise) that you want.
        d3.json(
            'https://docs.mapbox.com/mapbox-gl-js/assets/hike.geojson',
            function(err, data) {
                if (err) throw err;

                // save full coordinate list for later
                var coordinates = data.features[0].geometry.coordinates;

                // start by showing just the first coordinate
                data.features[0].geometry.coordinates = [coordinates[0]];

                // add it to the map
                map.addSource('trace', { type: 'geojson', data: data });
                map.addLayer({
                    'id': 'trace',
                    'type': 'line',
                    'source': 'trace',
                    'paint': {
                        'line-color': 'yellow',
                        'line-opacity': 0.75,
                        'line-width': 5
                    }
                });

                // setup the viewport
                map.jumpTo({ 'center': coordinates[0], 'zoom': 14 });
                map.setPitch(30);

                // on a regular basis, add more coordinates from the saved list and update the map
                var i = 0;
                var timer = window.setInterval(function() {
                    if (i < coordinates.length) {
                        data.features[0].geometry.coordinates.push(
                            coordinates[i]
                        );
                        map.getSource('trace').setData(data);
                        map.panTo(coordinates[i]);
                        i++;
                    } else {
                        window.clearInterval(timer);
                    }
                }, 10);
            }
        );
    });
</script>
